Vehicle wheel bearings include an inner race, often called a spindle, and a surrounding outer hub. Either the spindle or hub may be attached to the wheel, while the other is mounted to the vehicle suspension. The wheel may be powered or trailing. If trailing, then there is no need for a drive shaft to extend through the spindle, and the spindle can terminate at its inner end substantially even with the annular end face of the hub. It is most convenient to close that side of the bearing with a cup shaped end cap, stamped of metal or molded from a tough plastic material. The end cap is tightly press fitted to the hub, but does not touch the spindle, and so provides a complete seal with no friction and no moving interfaces to leak. The other side the bearing, where the spindle is attached to the rotating wheel, must be sealed with a rubbing seal.
The sealed internal volume of a wheel bearing is subject to pressure changes relative to the ambient or outside pressure. The bearing heats up as it runs, and also absorbs heat from the brake system, increasing the internal pressure. If the bearing blows off excess pressure while it is hot, then it can develop a pressure depression later, as it cools. It is desirable to equalize these pressure differentials, if possible. The rubbing seals in wheel bearings have flexible seal lips that make a line contact with a cylindrical seal surface. Since the seal lip is not attached to the sealing surface on which it rubs, it is fairly simple to mold some means into the seal lip, such as a fulcrum pad, to pry the lip up and vent excess pressure from the bearing. These designs are generally not able to breath in to relieve a negative pressure, however, since an air inflow would tend to shut the lip more tightly, rather than to lift it. Furthermore, the pads tend to wear away with time.
A typical end cap, which is stiff and tightly fitted to the hub, does not lend itself to the kind of lift and return venting that a flexible rubbing seal lip does. It would be possible to simply mount existing venting valves to the end cap, but that would add expense and extra manufacturing steps. An end cap that incorporated a vent means within its basic structure would be desirable, especially if it added no cost or complexity to the manufacture and installation of the end cap.